On large scales like the
Universe, the most important force is gravity. Between any two objects the
gravitational attraction is proportional to the product of the masses
divided by the square of the distance between them. Gravity is the force
responsible for keeping the Earth and other planets in our solar system in
orbit around the Sun. Gravity also governs the motions of the Sun and
nearly all the stars you can see in the sky, which are orbiting about the
centre of the Milky Way Galaxy. The Milky Way is part of a gravitationally
bound collection of galaxies which includes Andromeda, and is called the
Local Group. Apart from observing that objects large and small are
gravitationally attracted to each other, astronomers also observe that the
Universe is expanding: an after-effect of the birth of the Universe in the
Big Bang.

Size

Up until a year or two ago
it was thought that the Universe's expansion rate was decreasing, due to
gravity pulling back on the material exploding form the Big Bang. However,
more recently scientists have been able to measure how fast the Universe
was expanding, and the data indicate that it is actually speeding up. This
measurement was made by looking at distant supernovae. If distant
supernovae are different in brightness than nearby supernovae (e.g. if
there is more dust dimming the light than we think) then the measurement
could be wrong. However most astronomers think that the measurements are
strong.

The observable Universe is about 10
billion light years in radius. That number is obtained by multiplying how
old we think the Universe is by the speed of light. The reasoning there is
quite straightforward: we can only see out to that distance from which
light can have reached us since the Universe began.

We determine the age of the Universe in
a number of ways. One is to estimate the age of the oldest stars we see.
Our knowledge of how stars of a given size evolve with time is very good
(based on what we know about atomic and nuclear physics) so the major
uncertainty here is usually measuring how far away (and so how big) such
stars are. The standard method is to look for very small changes in the
apparent positions of the stars as the Earth moves around the Sun. (This
effect is called parallax). A second way to get an age for the Universe is
to try to figure out the time of the big bang itself. Here the method is
to use a series of techniques (based on how bright things appear to be -
like Cepheid variable stars - that we think we know the true brightness
of) to determine first the distance of the nearby galaxies, then
increasingly distant galaxies, until we have estimated distances for many
galaxies for which relative velocity measurements have been made (using
the Doppler red shift of features in their spectra). The relative
velocities we observe for distant galaxies have been largely determined by
the expansion of the Universe begun with the 'big bang'. So, once we've
determined how expansion velocity correlates with distance for some range
of distances, it's possible to extrapolate back (with some assumptions) to
calculate the instant of the big bang, when all the matter in the Universe
was at a single point.

We also observe that massive objects
attract each other through the gravitational force. This force tends to
contract matter locally (for example, a gas cloud condenses to form a
star). On the large scale you can think of the expansion of the Universe
acting to separate galaxies from one another, and the gravitational force
acting to attract them toward one another.

The end of time depends on just how much
mass there is in the Universe. We talk about this in terms of the density
of the Universe, and compare densities to the critical density. If the
density is greater than the critical density, then eventually gravity will
overtake the expansion. The expansion will slow down and eventually
reverse, so that the Universe will be contracting. Eventually it will end
in a collapse (or a bounce) called the Big Crunch. If the density is less
than the critical density then the Universe will continue to expand
forever, with the gravitational force never overtaking the expansion. An
ongoing area of research is to measure the density of the Universe.
Currently, some observations (and some theories) indicate that the density
of the Universe is very close to the critical density. In this case the
expansion will slow down so that it is approaching zero expansion as time
approaches infinity.

It has been said that because our
universe creates its own space and time it is expanding into pure nothing.
An infinitely dense point of matter appears in an otherwise perfect vacuum
state. An event of some sort causes that point to begin to expand very
rapidly, overcoming whatever initial gravitation pull would keep the point
of mass together. As the new universe continues to expand, there is less
and less gravitational pull to bring all of this mass back to its origin.
If the pulling force outward is constant and the gravitational pull
continues to decrease, the expansion rate will continue to increase. Apart
from that potentially damaging argument, there is the issue of the
definition of universe. Or universe, by one definition, is everything.
There is nothing beyond or outside of it, not even the empty space-time we
can conceive of as perfect space, so there would be no vacuum into which
the universe could expand. This may seem a bit of a paradox, as we can
always imagine something outside of our house or our solar system, but
then it really becomes a question of philosophy as much as science.

The second law of thermodynamics states,
that entropy will increase with time, where entropy = the amount of
disorder in a system. With increasing disorder, there is inherently less
energy that can be used to do useful work. With this inherent lack of
useful energy, at some point in time, the universe will reach a state of
thermal equilibrium, where there is nothing more than a collection of
protons evenly spaced apart and all moving at the same speed. That state
is called the heat death of the Universe. (The protons will all have
decayed, but the Universe will consist of smaller particles all drifting
at random and getting more and more distant from each other as the
Universe expands).

Dark
Matter

Dark
matter means just that; matter of whatever type that does not shine
brightly (in visual light, X-rays or at any other wavelengths). Even
though we do not see dark matter directly, its gravitational influence can
be seen in the motion of gas and stars in galaxies, and in the motion of
hot gas and galaxies within clusters of galaxies. Dark matter means matter
of an unknown type that astronomers and cosmologists believe must make up
the majority of the mass in the universe. Its existence was deduced from
the relative amounts of light elements and isotopes produced in the Big
Bang, from the properties of high-temperature gas located in clusters of
galaxies, and from the high speeds at which galaxies are moving in such
clusters. This means that there must be 10 to 20 times as much dark
matter, by mass, as ordinary matter, which scientists call baryonic
matter. There is recent evidence from microlensing observations that at
least some of the dark matter in our own galaxy is in the form of MACHOS,
or MAssive Compact Halo ObjectS. These are planets or stars, made up of
ordinary (baryonic) matter, that are too faint to be observed directly,
but can act as a gravitational lens and magnify the brightness of brighter
stars in the background. There is nothing weak in the observational proof
for dark matter in this sense.

In a cluster of galaxies, we can
estimate the masses of stars in the galaxies and the hot gas that fill the
cluster. We can also infer the total mass of the cluster that is needed to
keep it gravitationally bound. The latter is typically found to be ~5
times the combined mass of the stars and the hot gas; an analogy with our
Galaxy suggest that only some of the dark matter can be MACHOS. Although
circumstantial, such results point strongly to the presence of
non-baryonic dark matter in the clusters of galaxies. When it comes to
deciding what kind of exotic particles may make up the non-baryonic dark
matter, however, there may be a hint of weakness, in that different
particle physicists favor different exotic particles. Moreover, as far as
I know, there has not been a direct detection of these exotic particles.

Two likely possibilities for the dark
matter in our own galaxies are MACHOs (MAssive Compact Halo Objects) and
WIMPs (Weakly Interacting Massive Particles). MACHOs are low mass stars,
brown dwarfs, neutron stars and white dwarfs. If MACHOs make up most of
the dark matter, the distribution is not smooth on the scale of the Solar
system, but it is smooth on a much larger scale. If the Galactic dark
matter consists of WIMPs, then they are dispersed throughout the Galaxy,
with a distribution somewhat different from that of the stars that we can
see. Since gravitational pulls of WIMPs from different directions tend to
cancel out, the orbit of planets in our solar system is not affected by
the presence of WIMPs. However, since there are more WIMPs towards the
centre of our Galaxy than away from it, the motion of the Solar system
(and other stars) in the Galaxy is strongly affected. This is how
astrophysicists infer the presence of the dark matter.

One of the best ways of determining the
mass of a bound system, such as a cluster of galaxies, group of galaxies
or a massive elliptical galaxy , is to measure the X-ray temperature and
gas profiles. If the gas is in hydrostatic equilibrium, the total mass and
the gas mass can be directly determined from the X-ray data alone. Use of
this technique has shown that clusters of galaxies are gas and baryon
rich, that is, the mass in gas exceeds the mass in stars by factors of 3-5
and that the total baryonic mass is ~10-30% of the total mass of the
cluster. This discrepancy is called the baryon catastrophe. For galaxies
and groups, the X-ray data have often indicated very extended dark matter
halos far beyond the radius at which one sees starlight or galaxies. The
total inferred dark matter mass is often 10 times that in the visible
galaxies alone

Dark
Energy

Dark Energy appears to be
based on the brightness of the most distant type-Ia supernovae, a
mysterious force that is accelerating the expansion of the universe. These
recent discoveries have provided good evidence that there is such an
outward force on the universe (variously called the cosmological constant,
quintessence, or dark energy). Data about the rotation of galaxies shows
us that the outer parts rotate as fast as the inner parts. This only makes
sense if there is a spherical distribution of matter in each galaxy, which
is not what we see. Therefore we infer that there is a certain amount of
Dark Matter in each galaxy. This could be some exotic particles, or just
lots of stars too small to have ignited. Aside from this, there is also
the Dark Matter that we think is there, based on theoretical arguments.
This is something we can measure by looking at the cosmic microwave
background and distant supernovae. These are the measurements (recently
made) that imply the existence of both Dark Matter and Dark Energy

The
Great Attractor

The Great Attractor is far
bigger than a galaxy. In the terminology of astronomers, there are
clusters of galaxies containing maybe hundreds of galaxies, and
superclusters containing many clusters. The Great Attractor is a
supercluster or something even bigger. The gravity of the Great Attractor
has been pulling the Milky Way in its direction, the motion of local
galaxies indicated there was something massive out there that are pulling
the Milky Way, the Andromeda Galaxy, and other nearby galaxies towards it.
For a while, nobody could see what it was, because it lies behind the
plane of our Galaxy. That means the gas and dust in our Galaxy obscures
the light from the Great Attractor, and it is outshone by the stars and
other objects in our Galaxy. X-ray observations with the ROSAT satellite
then revealed that Abell 3627, a previously known cluster of galaxies, was
much more massive than originally suspected, containing many more
galaxies. Optical astronomers had missed a great number of galaxies,
because of the obscuration, but with hindsight (and with better
observations), could spot many more galaxies. It is now thought that the
Great Attractor is probably a supercluster, with Abell 3627 near its
centre.

Solar
Systems

A solar system is created
when a rotating cloud of gas and dust in space start to coalesce. They are
pulled together and towards the centre of the gas/dust cloud by their
gravitational attraction to each other. As they condense, the particles
collide faster and more often, which causes the gas and dust to heat up.
The gas and dust at the centre collapses to form the central star of the
solar system; the heat generated by the colliding particles starts nuclear
fusion in its core. If there was enough angular momentum in the system at
the very beginning, then not all of the dust and gas will go into the
central star. The rest will remain in a flattened disk around the star.
The planets form from this disk of rotating material as it clumps together
because of gravity.

Planets

As of August 24th 2006 the
International Astronomical Union decided that to be called a planet an
object must have three traits.

1) It must orbit the
sun,

2) be massive enough that its own gravity pulls it
into a nearly round shape, and

3) be dominant enough
to clear away objects in its neighbourhood.

To be admitted to the dwarf planet category an
object must have only two of those traits -- it must orbit the sun and
have a nearly round shape. And no, moons don't count as dwarf
planets.

In addition to Pluto, Ceres and 2003 UB313 the
astronomical union has a dozen potential 'dwarf planets' on its watchlist.
What's to become of the other objects in our solar system neighbourhood,
the ones that are not planets, not dwarf planets and not moons? The
organization has decided that most asteroids, comets and other small
objects will be called 'small solar-system bodies.' Despite the
establishment of these three distinct categories, there are bound to be
grey areas. As technologies improve and more objects are found the
International Astronomical Union will set up a process to decide which of
the three categories are most appropriate for specific objects.